1. Field of the Invention
This invention relates to a method of making a catalyst which is suitable for use as a three-way catalyst for auto emission control and to the resulting catalyst.
2. Description of the Previously Published Art
Even with the computer-controlled system, the success of the current single bed approach to the simultaneous control of HC, CO, and NO.sub.x emissions from automobile exhaust depends largely on how successfully one could design a practical three-way catalyst (TWC) with a large TWC "window". Because the size of TWC window is determined mainly by the CO conversion on the rich side (O.sub.2 -deficient) and the NO.sub.x conversion on the lean side (O.sub.2 -excess), suitable promoters that enhance these conversions will lead to a significant improvement in tne performance of catalytic converters.
The CO removal from automobile exhaust under rich conditions is highly dependent on the catalyst efficiency for the water-gas shift (WGS) reaction, CO+H.sub.2 O=CO.sub.2 +H.sub.2. It is known (G. Kim, "Ceria-Promoted Three-Way Catalysts for Auto Exhaust Emission Control," Ind. Eng. Chem. Prod. Res. Dev., 1982, 21, 267) that the WGS reaction is substantially enhanced on the supported Pt group metal TWCs when promoted by ceria which is moderately basic but which is also a strong Lewis acid. It is also known from Clyde L. Aldridge in U.S. Pat. No. 3,755,556 and Marnell A. Segura et al in U.S. Pat. No. 4,054,644 that the WGS reaction can be promoted by the presence of alkali or alkali earth metals which are strong bases. Ceria-promoted catalysts as taught by Sergeys, Maselli and Ernest in U.S. Pat. No. 3,903,020 and by Hindin and Dettling in U.S. Pat. No. 3,993,572 have been introduced for auto exhaust emission control. Alkali metal-promoted catalysts as taught by Baresel, Scharner, and Huth in U.S. Pat. No. 4,140,749 and by Barber, Lostaglio, and Shukis in their paper "Three Way Catalysts: Screening for Wide Window Performance," as presented at the 91st National Meeting of AIChE, Detroit, Michigan (August, 1981) have also been introduced for auto exhaust emission control. However, few catalysts have been tried with both alkali metals together with ceria.
The only publication found which uses alkali metals with ceria is the Toyota Motor Japanese patent Publication JP-004130 "Purification Catalyst for Motor Car Gas Exhaust" (1/18/80) which is U.S. Pat. No. 4,369,132. The problem they were addressing was to improve the low temperture performance of a catalyst that was mainly platinum. In other words, they were just concerned with improving the warm-up characteristic of the catalyst. The addition of ceria is said to be optional and there is no indication that any of the alkali metals are preferred. The testing undertaken was under mild conditions to only measure the temperature for 50% conversion. The patentees were not concerned with the problem of making a catalyst that would work well after operating for a substantial period of time. They did not measure the steady-state conversion efficiency nor did they use any accelerated aging techniques. Instead of using a high level poison containing fuel, they just used a commercial unleaded gasoline available from the market in their 30-hour aging in a multi-cell unit. Thus, their results cannot predict the expected performance of their catalysts after further extended period of aging beyond 30 hours. It should be pointed out that a catalyst with a decreased temperature for 50% conversion does not necessarily exhibit a high level of conversion under steady-state conditions. As will be shown in the following examples, the catalysts made according to the present invention have superior steady-state performance compared to the catalysts made according to this Toyota disclosure.
3. Objects of the Invention
It is an object of this invention to provide a method of producing a highly durable catalyst suitable for use as a three-way catalyst for auto emission control having good steady-state efficiency after a sufficiently long period of aging.
It is a further object of this invention to produce a catalyst which is doubly promoted with ceria and an alkali metal in a manner which permits the effective addition of noble metals to provide good steady-state efficiency when used for auto emission control.
It is a further object of this invention to produce a catalyst suitable for use as a three-way auto emission catalyst where ceria is initially deposited on the support followed by the addition of an alkali metal and one or more noble metals.
It is a further object of this invention to produce a catalyst suitable for use as a tnree-way auto emission catalyst where ceria and lithia are first deposited on a support followed by the addition of one or more noble metals.
These and further objects will become apparent as the description of the invention proceeds.